Abstract

The binding kinetics of several benzimidazole compounds were determined with recombinant tubulin monomers and heterodimers from benzimidazole-sensitive and -insensitive organisms. This study utilised the naturally occurring high efficacy of the benzimdazoles for the parasitic protozoa Giardia duodenalis and Encephalitozoon intestinalis. The benzimidazoles are not active against the protozoan Cryptosporidium parvum or mammalian hosts, including humans. The affinity of several benzimidazole derivatives for monomeric and heterodimeric beta-tubulin was clearly demonstrated, thus supporting previous studies of drug-resistant nematode and fungal populations. A homology model of protozoan alpha beta-tubulin, produced using the three-dimensionalstructure of mammalian alpha beta-tubulin, identified a strongly hydrophobic domain only on the beta-tubulin protein of sensitive protozoa. This domain is proposed to be the benzimidazole-binding domain and the amino acid residues within it include three key residues which are substituted between benzimidazole-sensitive and -insensitive organisms. These residues are Ile-189, Val-199, and Phe-200 that all have non-polar, hydrophobic side groups and are proposed to bind with the R5 side chain of several benzimidazole derivatives. In addition to this, the benzimidazole derivatives were able to bind irreversibly with assembling microtubules from sensitive parasites. The incorporation of benzimidazole-bound alpha beta-heterodimers into assembling microtubules was shown to arrest polymerisation in vitro although the addition of benzimidazole compounds to assembled microtubules did not result in depolymerisation. Taken together, these results suggest that the mechanism of action of these compounds is through disruption of the dynamic equilibrium that balances the cycle of microtubule polymerisation and disintegration within these protozoa. Further, this effect is brought about by preferential binding of the benzimidazoles to a hydrophobic region on the beta-tubulin protein.